The invention of the transistor and the integrated circuit has led to the development of improved methods for handling electronic devices. The semiconductor design and manufacturing process often results in specific devices having varying degrees of susceptibility to electrostatic discharge (ESD). When two objects with difference charges are brought together, the charges move between the objects until both have the same charge. The objects are thus discharged. The small amount of energy produced by an ESD can destroy the intricate pathways and gates inside an integrated circuit. ESD can be thought of in layman's terms as a static spark.
Despite a great deal of effort, ESD still affects production yields, manufacturing costs, product quality, product reliability, and profitability. The cost of damaged devices themselves ranges from only a few cents for a simple diode to several hundred dollars for complex hybrids. When associated costs of repair and rework, shipping, labor, and overhead are included, the costs may be much higher.
Static electricity is generally defined as an electrical charge caused by an imbalance of electrons on the surface of a material. This imbalance of electrons produces an electric field that can be measured and that can influence other objects at a distance. ESD is generally defined as the transfer of charge between bodies at different electrical potentials.
Electrostatic discharge can change the electrical characteristics of a semiconductor device, degrading or destroying it. Electrostatic discharge also may upset the normal operation of an electronic system, causing equipment malfunction or failure. Another problem caused by static electricity occurs in clean rooms. Charged surfaces can attract and hold contaminants, making removal from the environment difficult. When attracted to the surface of a silicon wafer or a device's electrical circuitry, these particulates can cause random wafer defects and reduce product yields.
ESD can be protected against by at least two approaches. First, environmental considerations can be taken into account to reduce the potential for ESD as much as possible. Second, a semiconductor device can intrinsically have some ESD protection. Usually, a combination of at least these two approaches is used, such as by designing a certain level of ESD resistance in a semiconductor device, and instructing users to take basic ESD precautions.
An example of designing ESD protection into a semiconductor device is the guard ring, or guard track. A guard ring does not carry a return current for a semiconductor circuit under normal operation, and is tracked around the entire perimeter of the semiconductor device, or around individual circuits or components of the device. The purpose of the guard ring is to act as a return source for current that is radiating out of or incident to the semiconductor device.
Guard rings are also used in the context of semiconductor photolithographic masks employed during semiconductor device fabrication. A mask, or a photomask, is a device that allows selective exposure of photoresist on a semiconductor wafer by blocking exposure in certain areas. A mask includes two parts, an opaque, or non-transparent, part, and a blank, or transparent, part. Generally, the blank part of the mask is the actual substrate of the mask, such as high-quality quartz or glass. The opaque part is then formed on the blank part, and may be made of chrome. Guard rings are thus placed on photomasks to protect the important circuit patterns of the masks from being damaged by ESD. Guard rings in such instances may track around the perimeter of the entire mask, or only around crucial circuits on the mask.
FIG. 1 shows an example of a photomask 100 that has a guard ring 101 formed on it. The substrate of the mask 100 is transparent, and can be quartz or glass. The guard ring 101 is opaque, and can be chrome. The guard ring 101 has an outer ring 102 and an inner ring 104. The rings 102 and 104 may initially have a breakdown voltage against which they protect. That is, the rings 102 and 104 protect against ESD by being rated for a certain breakdown voltage. This rating ensures that, initially, the rings 102 and 104 can accommodate an ESD of their associated rated voltage, and thus protect the circuits that the rings 102 and 104 circumscribe (not particularly shown in FIG. 1).
A particular area 107 of the guard ring 101 is shown in more detail in FIG. 2. Each of the outer ring 102 and the inner ring 104 has a number of lightening bars. For example, one of the lightening bars of the outer ring 102 is indicated as the lightening bar 106, whereas one of the lightening bars of the inner ring 104 is indicated as the lightening bar 108. As shown in FIG. 2, each of the rings 102 and 104 is a solid ring, such that the lightening bars of the ring 102 are electrically connected to one another, and the lightening bars of the ring 104 are electrically connected to one another.
As shown in FIG. 2, the inner ring 104 has protected against the breakdown voltage, as evidenced by one of its lightening bars, the lightening bar 110, having shorted with the outer ring 102. Whereas this ESD protection has protected the semiconductor circuits imprinted on the mask 100 of FIG. 1, the guard ring 101 is seriously impaired for future ESD protection. In particular, the shorting of the lightening bar 110 causes the outer ring 102 to now be electrically connected to the inner ring 104. This means that the there may be no ESD protection afforded by the outer ring 102 and the inner ring 104. Thus, the internal circuitry imprinted on the mask 100 may no longer be protected.
Therefore, there is a need a guard ring that overcomes these disadvantages. Specifically, there is a need for a guard ring that still allows for ESD breakdown voltage protection, even after one of the outer and inner rings of the guard ring has already protected against ESD. For these and other reasons, there is a need for the present invention.